This application pertains to the art of video display devices and more particularly to video display devices used to display readout images from medical diagnostic equipment. The invention is particularly applicable for displaying the pictures derived from data obtained by scintillation cameras also commonly referred to as radioisotope, gamma, nuclear or Anger-type cameras. Although the invention will be described with particular reference to scintilltion cameras, it will be appreciated that the invention has much broader application.
A persistence scope is a cathode ray tube on which excited regions of phosphor persist or remain luminescent for an extended period of time before fading into darkness. Heretofore, the persistence of luminescing regions on the screen of cathode ray tubes has been achieved through the use of a phosphor which continues to luminesce after the electron beam excitation has been removed. Normally, the rate of fading is determined by the phosphor and is adjusted by controlling the amount of excitation energy in the electron beam.
One of the prior art problems has been the lack of ability of control differences in displayed brightness or intensity. It is an inherent limitation of persistence cathode ray tubes that only three or four discernible shades of grey are displayable. The lack of distinctive levels of shading make output data relatively difficult to read and interpret. In scintillation cameras, for example, differences in intensity are important indicia to the medical technician of variation in radioisotope concentration. It is desirable to be able to determine these concentrations accurately.
Another problem with the prior art persistence screens is size and cost. Persistence screens are costly to manufacture even in small sizes and cost increases rapidly with size. Commonly, only relatively small persistence screen sizes are available rendering interpretations of the display relatively inaccurate.
Further, persistence cathode ray tubes are electronically complex. This complexity results in unreliability and difficulty of adjustment. It is not uncommon for prior art persistence scopes to fail at a rate approaching thirty percent per year.